Electronic spectra, resonance raman spectra and solution properties of water-soluble Cu(II), Ni(II) and Co(III) porphyrins

Effects of Temperature, Axial Ligand, and Photoexcitation on the Structure and Spin-State of Nickel(II) Complexes with Water-Soluble 5,10,15,20-Tetrakis(1-methylpyridinium-4-yl)porphyrin

Water-soluble metalloporphyrins, depending on the metal center, possess special spectral, coordination, and photochemical features. In nickel(II) porphyrins, the Ni(II) center can occur with low-spin or high-spin electronic configuration. In aqueous solution, the cationic nickel(II) complex (Ni(II)TMPyP4+, where H2TMPyP4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin), exists in both forms in equilibrium. In this study, an equilibrium system involving the low-spin and high-spin forms of Ni(II)TMPyP4+ was investigated via application of irradiation, temperature change, and various potential axial ligands. Soret band excitation of this aqueous system, in the absence of additional axial ligands, resulted in a shift in the equilibrium toward the low-spin species due to the removal of axial solvent ligands. The kinetics and the thermodynamics of the processes were also studied via determination of the rate and equilibrium constants, as well as the ΔS, ΔH, and ΔG values. Temperature increase had a similar effect. The equilibrium of the spin isomers was also shifted by decreasing the solvent polarity (using n-propanol) as well as by the addition of a stronger coordinating axial ligand (such as ammonia). Since triethanolamine is an efficient electron donor in Ni(II)TMPyP4+-based photocatalytic systems, its interaction with this metalloporphyin was also studied. The results promote the development of efficient photocatalytic systems based on this complex.

Molecular Interactions Identified by Two-Dimensional Analysis-Detailed Insight into the Molecular Interactions of the Antimalarial Artesunate with the Target Structure β-Hematin by Means of 2D Raman Correlation Spectroscopy

A thorough understanding of the interaction of endoperoxide antimalarial agents with their biological target structures is of utmost importance for the tailored design of future efficient antimalarials. Detailed insights into molecular interactions between artesunate and β-hematin were derived with a combination of resonance Raman spectroscopy, two-dimensional correlation analysis, and density functional theory calculations. Resonance Raman spectroscopy with three distinct laser wavelengths enabled the specific excitation of different chromophore parts of β-hematin. The resonance Raman spectra of the artesunate-β-hematin complexes were thoroughly analyzed with the help of high-resolution and highly sensitive two-dimensional correlation spectroscopy. Spectral changes in the peak properties were found with increasing artesunate concentration. Changes in the low-frequency, morphology-sensitive Raman bands indicated a loss in crystallinity of the drug-target complexes. Differences in the high-wavenumber region were assigned to increased distortions of the planarity of the structure of the target molecule due to the appearance of various coexisting alkylation species. Evidence for the appearance of high-valent ferryl-oxo species could be observed with the help of differences in the peak properties of oxidation-state sensitive Raman modes. To support those findings, the relaxed ground-state structures of ten possible covalent mono- and di-meso(Cm)-alkylated hematin-dihydroartemisinyl complexes were calculated using density functional theory. A very good agreement with the experimental peak properties was achieved, and the out-of-plane displacements along the lowest-frequency normal coordinates were investigated by normal coordinate structural decomposition analysis. The strongest changes in all data were observed in vibrations with a high participation of Cm-parts of β-hematin.

Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions

Finding photostable, first‐row transition metal‐based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)₃]²⁺ as photosensitizer and [S₂O₈]²⁻ as sacrificial electron acceptor. Electron‐poorer Ni‐porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron‐poorer porphyrins afforded 6‐fold higher turnover frequencies (TOFs; ca. 0.65 min⁻¹) than electron‐richer analogues. However, the electron‐poorest Ni‐porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O₂ evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)₃]³⁺. Critically, these Ni‐porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3 % less O₂ than during the first photocatalytic run.

Coordination-induced spin-state switching with nickel chlorin and nickel isobacteriochlorin

We present the first coordination-induced spin-state switching with nickel chlorin and nickel isobacteriochlorin. The spin-state switching was monitored by UV-vis spectroscopy and NMR titration experiments. The association constants (K1 and K2) and thermodynamic parameters (ΔH and ΔS) of the coordination of pyridine were determined. The first X-ray analyses of a paramagnetic nickel chlorin and a nickel isobacteriochlorin provide further information about the structure of the octahedral complexes. Nickel chlorin and even more pronounced nickel isobacteriochlorin exhibit stronger coordination of axial ligands compared to the corresponding nickel porphyrin and thus provide the basis for more efficient spin-switching systems.

Coordination-Induced Spin-State-Switch (CISSS) in water

A water-soluble Ni-porphyrin switches the spin state and proton relaxation time of the surrounding water upon addition of an axial ligand.

Spectral investigations of the solution properties of 5,10,15,20-tetrakis(4-N-benzyl-pyridyl)porphyrin (TBzPyP) and its interaction with human serum albumin (HSA)

The association behavior of 5,10,15,20-tetrakis(4-N-benzyl-pyridyl)porphyrin ( TBzPyP ) was investigated in aqueous solution at 27 °C and various ionic strengths by optical absorption, fluorescence and resonance light scattering (RLS) spectroscopies. The results show that TBzPyP exists as monomer at low ionic strength and as ill-defined aggregates at high ionic strength and does not show concentration dependent aggregation over an extended concentration range between 5 × 10−7 to 1 × 10−4 M in water. The interaction of the TBzPyP with human serum albumin (HSA) in 5 mM phosphate buffer, pH = 7.00, and at 27 °C, has been also studied by optical absorption, fluorescence and RLS spectroscopies. The formation of two types of complexes (HSA: TBzPyP and (HSA)2: TBzPyP ) has been demonstrated by analysis of optical absorption spectral patterns of TBzPyP at various concentrations of HSA. Based on absorption data, a binding model has been proposed and used to analyze the binding process. The calculated binding constant for formation of HSA: TBzPyP and ( HSA )2: TBzPyP are 3.79 × 106 M−1 and 1.46 × 105 M−1, respectively. The RLS spectra of TBzPyP at various concentration of HSA do not show any aggregation of TBzPyP in the presence of HSA. Due to the fact that binding of TBzPyP quenches the HSA fluorescence, a step-by-step aggregation model, which has been developed by Borissevich, I. E. et al, was used to determine the average aggregation number of HSA, < J >, from the fluorescence quenching. The < J > value is 1.4, which further confirms the formation of the two mentioned complexes at this concentration range of HSA.

Polystyrene sulfonate-porphyrin assemblies: influence of polyelectrolyte and porphyrin structure

In this study, electrostatic self-assembly of different polystyrene sulfonates and a set of tetravalent cationic porphyrins is investigated. It is shown that association of linear polystyrene sulfonates of different molar masses yields finite size nanoscale assemblies that are stable in aqueous solution. Aggregates are compared to the ones of cylindrical brushes, revealing that both form assemblies in the 100 nm range with the charge ratio (molar ratio of porphyrin charges to polyelectrolyte charges) being determining, while the morphology of the resulting network-like assemblies is different for both polyelectrolyte architectures. For the smallest 8k polystyrene sulfonate, in addition, stoichiometric conditions differ. The influence of the molecular porphyrin structure was investigated by comparing meso-tetrakis(4-(trimethyl-ammonium)phenyl)porphyrin (TAPP) with its Cu(II) and Zn(II) loaded analogues and meso-tetrakis(4-N-methylpyridinium)porphyrin (TMPyP), revealing differences in stacking tendency and geometry. Additionally, the TMPyP accumulates more in the inside of the brush than the other porphyrins, likely due to the different position of its charged groups. The supramolecular nanostructures formed were characterized by UV-vis spectroscopy, light scattering, atomic force microscopy, cryo transmission electron microscopy, and small-angle neutron scattering. Results may build a valuable basis for the use of polyelectrolyte-porphyrin assemblies in medicine, catalysis, or energy conversion.

Equilibrium, photophysical, photochemical, and quantum chemical examination of anionic mercury(II) mono- and bisporphyrins

Mercury(II) ion and 5,10,15,20-tetrakis(parasulfonato-phenyl)porphyrin anion can form 1:1, 2:2, and 3:2 (metal ion/porphyrin) out-of-plane (OOP) complexes, from which Hg2P2(8-) has not been identified until now. Identification of this species significantly promoted the confirmation of the composition and the precise elucidation of the equilibrium of Hg3P2(6-). Since the formation of each complex is too fast, their kinetic behavior was studied from the side of dissociation. The rate-determining step in dissociations, as well as in the formation of the 2:2 complex, that is, the dimerization of 1:1 complex, proved to be virtually first-order under these conditions, while the consecutive formations of HgP(4-) and Hg3P2(6-) are second-order reactions. The equilibria can be spectrophotometrically investigated because the Soret- as well as the Q-absorption bands of the free-base ligand are more and more red-shifted in the series of 1:1, 2:2, and 3:2 complexes, and the split of Q-bands disappears as the singlet-1 excited states become degenerate; in the case of bisporphyrins, the bands broaden, especially in the longer-wavelength region of the spectra. The quantum yield and the lifetime of S1-fluorescence from the macrocycle is decreased by the insertion of a mercury(II) ion due to distortion, and in bisporphyrins the luminescence totally ceases because their more complicated structure promotes other ways of energy dissipation. The lifetime of the triplet excited-state is also reduced by metalation. The transient absorption measured upon excitation of Hg3P2(6-) probably originates from Hg2P2(8-) formed by efficient photodissocation during the laser pulse. This photoinduced dissociation is characteristic to out-of-plane complexes, but in metallo-monoporphyrins it needs the energetically higher Soret-excitation; in bisporphyrins, it can take place during irradiation at the longer Q-wavelengths. Investigation of the intramolecular photoredox reactions has proved that for the increased efficiency of the indirect photoinduced LMCT, not the redox potential, but the position of the metal center is responsible. The two orders of magnitude higher photoredux quantum yield for the 3:2 complex, compared to that of the 2:2 species, can be explained by the repulsive effect of the inner mercury(II) ion pushing the other two farther out of the ligand cavity. In bisporphyrins the second excited states are photochemically more reactive than the first ones, while most of the photochemical processes of HgP(4-) originate from the first excited state. According to our quantum chemical calculations, the mercury(II) ion causes the expansion of the porphyrin-cavity; therefore its out-of-plane position is smaller than the value expected based on its ionic radius. In the hitherto unknown 2:2 dimer two 1:1 saucer-shaped monomers are kept together by secondary forces, mostly by pi-pi interaction, but their relative arrangement was not unequivocally determined by the two DFT functionals used. The arrangements with a symmetry axis or plane perpendicular to both rings are not favored; instead, the two monomers are shifted along the porphyrin planes, either in a Hg-P-Hg-P or a Hg-P-P-Hg order. Our time-dependent density functional theory (TD-DFT) calculations indicate that the electronic spectra are not very sensitive to the structure of the dimer, even though the environment of the porphyrin rings is quite different if one of the metal ions is between or outside of both macrocycles. The calculated spectral shifts agree only partially with the experimental data. The TD-DFT calculations suggest that the chromophores are not fully independent in the bisporphyrins and that the observed spectral shift cannot be uniquely assigned to the geometrical distortion of the porphyrin macrocyle.

The DNA-Porphyrin Interactions Studied by Vibrational and Electronic Circular Dichroism Spectroscopy

The interactions of three different porphyrins, without axial ligands - 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin-Cu(II) tetrachloride (Cu(II)TMPyP), with axial ligands - 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin-Fe(III) pentachloride (Fe(III)TMPyP), and with bulky meso substituents - 5,10,15,20-tetrakis(N,N,N-trimethylanilinium-4-yl)-porphyrin tetrachloride (TMAP), with calf thymus DNA were studied by combination of vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopy, and by IR and UV-VIS absorption spectroscopy. It has been shown that Cu(II)TMPyP prefers the intercalative binding mode with DNA in the GC-rich regions and the intercalative sites are saturated at the c(DNA)/c(Cu(II)TMPyP) ratio ~3:1, where c(DNA) and c(Cu(II)TMPyP) are total molar concentrations of nucleic acid in base pairs and porphyrin, respectively. Fe(III)TMPyP does not intercalate between the GC base pairs but binds to DNA in the minor groove. At higher c(DNA)/c(TMAP) ratios, TMAP interacts with DNA in the minor groove, but at lower ratios in the major groove and by the external binding mode accompanied by self-stacking of porphyrins along the phosphate backbone. VCD spectroscopy reliably discriminates the binding modes and specifies the conformational changes of the DNA matrices. It has been also shown that VCD spectroscopy is an effective tool for the conformational studies of DNA-porphyrin complexes. New spectroscopic "markers" in VCD spectra have been found for the specific DNA-porphyrin interactions.

Self-aggregation inhibits the photonuclease activity of porphyrins

A series of tentacle porphyrins having four aminoalkyl groups at the periphery was synthesized, and the DNA binding properties were investigated by absorption and circular dichroism (CD) spectroscopic methods. The aminopropyl chain was found to facilitate binding, and bisignate induced CD spectra revealed that the porphyrins are self-stacked on the DNA surface. The photonuclease activity of the tentacle porphyrins was also studied, and the aminopropylporphyrin showed the highest activity. The activity increased in proportion to the porphyrin load, but higher loads resulted in the decrease of activity. This inhibitory step corresponded to aggregation of the porphyrin. Thus, the aggregation was suggested to shield the inner porphyrin from the solvent, the production of active oxygen species being suppressed.

Raman and infrared spectral study of meso-sulfonatophenyl substituted porphyrins (TPPSn, n = 1, 2A, 2O, 3, 4)

Raman and IR spectra of the free base p-sulfonatophenyl and phenyl meso-substituted porphyrins [5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4); 5,10,15-tris(4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS3); 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS2A); 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS2O); and 5-(4-sulfonatophenyl)-10, 15,20-trisphenylporphyrin (TPPS1)] and their N-diprotonated derivatives (porphyrin diacids) were studied. The Raman spectra of the deuterated analogues of these porphyrins, in which the central hydrogen atoms were substituted with deuterium, were also measured. The observed vibrational bands were assigned on the basis of the deuteration shifts and compared with the structural analogues of these compounds. In IR spectra of the free-base porphyrins, the p-sulfonation of phenyl groups results in evident alteration for the phenyl modes and the porphyrin skeleton modes that are strongly coupled with phenyl vibrations. While the p-sulfonation of phenyl groups causes only slight changes for the high-frequency Raman bands (> 850 cm(-1)), dramatic shifts and band splitting were observed in the low-frequency region (< 500 cm(-1)) of Raman spectra. The observed differences of low-frequency Raman spectra were attributed to the alteration of the structure of the porphyrin ring, especially the CalphaCmCalpha bond-angles, by different meso-sulfonatophenyl substitutions. In addition, different packing style of TPPSn molecules in the aggregates is also responsible for the alteration of the vibrational spectra of the aggregated TPPSn.

Copper insertion facilitates water-soluble porphyrin binding to rA.rU and rA.dT base pairs in duplex RNA and RNA.DNA hybrids

The binding of the copper(II) complex of water-soluble meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) to double-helical polynucleotides has been studied by optical absorption, circular dichroism (CD), and resonance Raman spectroscopic methods. The target polymers were RNA and RNA.DNA hybrids consisting of rA.rU, rI.rC, rA.dT, and rI.dC base pairs. Relative to the metal-free H(2)TMPyP [Uno, T., Hamasaki, K., Tanigawa, M., and Shimabayashi, S. (1997) Inorg. Chem. 36, 1676-1683], CuTMPyP binds to poly(rA).poly(dT) and poly(rA).poly(rU) with a greatly increased binding constant. The external self-stacking of the porphyrin on the surface of the polymers was evident from the strong conservative-type induced CD signals. The signal intensity correlated almost linearly with the number of stacking sites on the polymer except for poly(rA).poly(dT), which showed extraordinarily strong CD signals. Thus, the bound porphyrin may impose an ordered architecture on the polymer surface, the stacking being facilitated by the more planar nature of the CuTMPyP than the nonmetal counterpart. Resonance Raman spectra of the stacked CuTMPyP were indistinguishable from those of the intercalated one with positive delta(Cbeta-H) and negative delta(Cm-Py) bending shifts, and hence the stacked porphyrins are suggested to adopt a similar structure to that of intercalated ones. Porphyrin flattening by copper insertion opens a new avenue for medical applications of porphyrins, blocking biological events related to RNA and hybrids in malignant cells.

Binding of meso-Tetrakis(N-methylpyridinium-4-yl)porphyrin to Double Helical RNA and DNA.RNA Hybrids

The binding properties of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H(2)TMPyP) to RNA and DNA.RNA hybrid duplexes were studied by absorption and circular dichroism (CD) spectra. The duplexes studied were poly(rA).poly(rU), poly(rA).poly(dT), poly(rI).poly(rC), poly(rI).poly(dC), poly(rG).poly(rC), and poly(rG).poly(dC). The hypochromicity (about 40%) and the bathochromic shift (about 15 nm) of the porphyrin Soret absorption band upon binding were quite similar among the duplexes examined. The large bathochromic shift and hypochromicity suggested a significant perturbation in the porphyrin pi electrons upon binding. H(2)TMPyP was found to bind in a single step to poly(rI).poly(rC), poly(rG).poly(rC), and poly(rG).poly(dC) and in a multistep manner to poly(rA).poly(rU), poly(rA).poly(dT), and poly(rI).poly(dC). The induced CD spectra in the visible range suggested that the porphyrin preferred to bind to the RNA duplexes with self-stacking along the polymer surface and to the hybrids with intercalation, at least at higher duplex load. This implied a distinct conformational difference between the RNA duplexes and DNA.RNA hybrids, and a drug molecule is able to recognize the difference. The number of binding sites per base pairs (n), however, was very different among the RNA duplexes examined. We also found that the intensity of the bisignate-induced CD bands is proportional to the n value. This suggested that the transition moments on the neighboring porphyrins are interacting considerably with each other to produce intense induced CD peaks.